The present invention relates to a technology of forming a specimen of a transmission electron microscope (TEM specimen) into a thin specimen by a focused ion beam.
It is already known to prepare a thin specimen, for a cross-sectional observation by a transmission electron microscope (TEM), by a thin film working with a focused ion beam (FIB) apparatus, and there are known (1) a method of executing an etching work in a wafer state thereby obtaining a thin film specimen formed into a thin piece, and (2) a method of mechanically cutting out a small piece from a wafer-shaped specimen and working on such piece.
In the method (1), as shown in FIGS. 4A–4D, a portion desired for cross-sectional observation is specified in a large specimen such as a wafer, then an FIB irradiation is executed under a blowing of a raw material gas from a gas gun to apply a protective deposition film, then holes are formed by an FIB etching on both sides of such cross section, further the cross-sectional portion for observation is worked into a thin piece and then cut in periphery, and the thin specimen is lifted out by a glass probe through an operation of a manipulator (not shown) and is fixed on a mesh of an organic thin film to constitute a TEM specimen.
In case of preparing a cross-sectional TEM specimen by the method (2), at first, on a small-piece specimen shaped into several tens of micrometers by a mechanical polishing as shown in FIG. 5A, an FIB irradiation is executed under a blowing of a raw material gas by a gas gun to apply a protective deposition film in front of and behind a portion to be observed, as shown in FIG. 5B. In this specimen, an FIB etching work is executed to leave a wall of 0.5 μm or less as shown in FIG. 5C. Then the specimen after the working is observed under a TEM as shown in FIG. 5D to confirm whether the working is done to have an appropriate thickness. In case the working is insufficient, the ion beam etching work has to be executed again. This method, requiring to transfer the specimen between plural vacuum apparatuses, namely the FIB apparatus and the TEM apparatus, is associated with drawbacks of requiring a time for evacuation and positioning of the specimen, and of difficulty in preparing an optimum cross-sectional TEM specimen.
In order to resolve such drawbacks, the present applicant already proposed a “focused ion beam apparatus and a working observing method” in JP-A No. 6-231720. A focused ion beam apparatus for working, disclosed in the patent, is provided as shown in FIG. 6 with an electron lens barrel (electron gun 6, electron optical system 8) in addition to an ion lens barrel (ion gun 1, ion optical system 3), and characterized in including an irradiation system for irradiating the specimen with an electron beam from a lateral direction and detectors 5, 9, 11, 10 for detecting electron beam-excited secondary signals (secondary electrons, reflected electrons, transmitted electrons, Auger electrons and X-ray), and is capable of scanning with the FIB 2 and detecting ion beam-excited secondary electrons to observe an image of a scanning ion microscope (SIM) thereby determining a working observing position of a specimen, and then executing an ion beam etching work on the surface of the worked specimen, thereby achieving a thin piece formation of a specified position of the specimen, particularly preparation of a cross-sectional TEM specimen. It is also possible to switch the ion beam to an aforementioned electron beam whenever necessary thereby observing a working state by a SEM image or monitoring reflected electrons, transmitted electrons, or an X-ray excited by transmitted and scattered electrons, thereby estimating the thickness of the thin specimen. This apparatus enables to work a specified part of the specimen into a thin piece by FIB etching and to switch the ion beam into the electron beam whenever necessary during the working operation without changing the apparatus whereby a SEM image observation or an X-ray analysis can be executed to easily achieve a confirmation of a working position, a working shape or a cross section or an analysis of a small portion, and a monitoring of reflected electrons, transmitted electrons or transmitted and scattered electrons allows to check the thickness of the thin-worked specimen by an electron transmittance thereof instead of a mere confirmation of the dimension, and thus provides an advantage that an appropriate thin film working can be easily executed. In the aforementioned apparatus, however, though the transfer of the specimen between the different apparatus, namely between the FIB apparatus and the electron microscope is unnecessary, it is still necessary, in order to execute the necessary thin film working, to interrupt the working in the course thereof, then to confirm the film thickness by the STEM image observation in the lateral direction, and repeating the working and the confirmation of film thickness, thus requiring works by the operator and a working time.
Under such situation, there is being adopted, in case of working specimens of a same structure in a same material, a method of preparing a working recipe by confirming the working conditions in advance and executing the working thereafter based on such recipe thereby obtaining an approximately required thickness. There is also being developed an apparatus for automatic working by programming such method, and the TEM specimen working that has required a professional skill can now be simply achieved even to an unskilled person by such programmed working. However, the preparation of such working recipe requires a skill as in the past and also requires a working time corresponding to several TEM specimens. Therefore, in case the number of the specimen is limited, the specimen working is being executed in the prior method since the recipe preparation requires more time. Also the method of estimating the film thickness of the specimen in the course of FIB working, utilizing the signal of electrons reflected or transmitted by the thin film or X-ray induced by the electrons, is acceptable for a specimen bearing no pattern and having a similar structure in any position, but, in case of a device having a complex cross-sectional structure such as an LSI, it is still necessary to interrupt the operation in the course of the working, then to confirm the film thickness based on the signal from the cross section and to re-start the working, and to thereafter repeat the interruption of the working and the confirmation of the film thickness, thus necessitating a time for the working.
An object of the present invention is to provide a method capable of achieving a control of a set film thickness by the FIB working without a working recipe for thin film working and enabling a fine working such as of a TEM specimen in a simple manner even with a skill of a certain level, and a system for executing such method.